The board of directors of any employers' mutual liability insurance association may, from time to time, fix and determine the amount to be paid as dividends upon policies expiring each year, after retaining the unearned premiums upon undetermined risks and sufficient sums to pay all the compensation then payable or which may become payable on account of injuries theretofore received by employes of the subscribers, and to pay the expenses incurred in the operation of the business of the association, and such percentage of the premiums as have been paid or are payable to create and maintain the surplus provided in section six hundred and forty-three (643).
40 P.S. § 787